Salim bin Amir reports that there was a treaty for a certain period between Muawiyah (may Allah be pleased with him) and the Romans. Muawiyah (may Allah be pleased with him) used to go towards their cities so that when the term of the treaty expired, he could attack them. Suddenly, he saw a man on his mount or horse saying: "Allahu Akbar! You should fulfill the covenant on your part, not break it." That man was Amr bin Abasah (may Allah be pleased with him). So Muawiyah (may Allah be pleased with him) asked him about this, and he said: "I heard the Messenger of Allah (peace and blessings be upon him) say: 'If there is a treaty between a man and a people, he should not break the treaty or make a new one until its term has expired or he returns it to them on equal terms.'" Muawiyah (may Allah be pleased with him) then returned with the people. Imam Tirmidhi says:
This hadith is Hasan Sahih.